Marine heatwave pushing tropical fish into Northland waters

Recreational fisher Rommel Laya was hoping to land a snapper but hooked a baby shark off a jetty in Onerahi. Photo / Tania Whyte

Fishers and divers are having a fintastic time as warmer than normal sea temperatures are pushing more fish from the tropics into Northland waters.

A marine heatwave is happening all around New Zealand and Niwa said people should expect more species such as marlin, mahi-mahi, and little tropical vagrant species to turn up in Poor Knights Islands.

Marine heatwaves are defined as periods of five days or more of ocean temperatures in the top 10 per cent of local average values for the time of year.

"Sea temperatures have been 4C above average in some coastal areas and it looks like warm seas are going to be with us throughout the summer season," Niwa forecaster Nava Fedaeff said.

She said La Niña in the Pacific, which tended to cause more subtropical, northeasterly winds, and high air pressures, was bringing warm air temperatures and high humidity.

Fisheries scientist Darren Parsons said more tropical and subtropical fish that turned up in Northland waters would be a delight for fishers and divers.

"Water temperatures also impact a fish's metabolism. Their body temperature is the same as the water around it, so when they're warmer, their metabolism speeds up. This means they need to eat more.

"This is great news for fishers - getting a good catch might be a lot easier this year. If the fish are hungry, they're more likely to eat your bait."

Troy Makiri casts his line off a jetty on Wharf Rd in Parua Bay. Photo / Tania Whyte

On Thursday the water temperature at a depth of 41m off Cavalli Islands, north of Kerikeri, was 21C, a temperature typical for late February or March.

Whangārei man Troy Makiri cast his line off a jetty on Wharf Rd in Parua Bay yesterday and said he caught two fish off Onerahi a year ago he thought were not typically found in New Zealand waters.

"They looked like a baby ling and likely came from tropical waters. Marine life is good, it's healthy and you can catch what you couldn't catch two years ago," he said.

Aucklander Peter Olliphant was casting his line from the same jetty he used to frequent while growing up in Kamo.

"The water's definitely warmer. In those days, you'd get more fish close to the shore than you do now. I used to go diving for paua but these days, you have to go out a bit deeper."

Aucklanders Peter Olliphant, left, David Melon, Willy Shum, Logan Olliphant and Paris Shum ready to go fishing at Parua Bay. Photo / Tania Whyte

Whangārei-based marine ecologist Irene Middleton said divers in particular were finding wrasses and damsel and as waters got warmer, species that typically survived in cold temperatures would move south or to deeper waters.

"The biggest change we are noticing is the heatwaves are keeping ocean temperatures warm enough over winter so tropical fish are able to survive. The tropical reef fish don't survive if temperatures drop to 15 or 16 degrees but some are starting to adapt, especially in the Far North."

Middleton said sightings of tropical fish species meant New Zealand waters were getting diverse but were potentially bad for those marine lives that were getting stressed by warm temperatures.

She said warm currents from Norfolk Island and Lord Howe Island pushed right into Northland shores where tropical fish started to make their habitat.

There were quite a lot of mahi-mahi in waters off the Far North coast, she said, while fishers in Taranaki were catching marlin earlier than expected.

Jeroen Jongejans of Dive! Tutukaka said tropical species such as manta rays and sunfish have been seen in waters between the Poor Knights Islands and the mainland.

"Big blue waters between the Poor Knights and the mainland with temperatures of 21 or 22 degrees and around 18 degrees around the coast is absolutely exquisite and it's an exciting time of the year.

"We've had a lot of dolphins, whales and when the weather is good, people are out on the water and they see things they normally don't see," he said.

Jongejans said a downside of warmer waters was chances of getting tropical cyclones.
Parsons said shark sightings could increase too at this time of the year.

"If it's nice weather and the waters are pleasant, more people will be out surfing and swimming. Just by being out in the water more, we're more likely to see sharks. But sharks head inshore during the summer season anyway.

"Some species follow the warm water to feed and give birth to live young. There may be more than usual because it's a marine heatwave, but we can't say for sure. What we do know is that they will be there."

Marine ecologist Irene Middleton said some tropical fish were beginning to adapt to winter temperatures in Northland waters. Photo / Supplied

In terms of daily catch limits on tropical fish, a Ministry of Primary Industries' spokesman said if the species people caught were not in the quota management system, then those rules didn't apply.

Tropical fish were unlikely to be added to the system if warmer than expected sea temperatures did not happen on a regular basis, he said.

La Niñas and higher water temperatures correlate with large spawning events for some species. Snapper, for example, do their spawning migrations in spring when the water reaches about 16C, and in the past warmer years have led to more successful spawning.

In a La Niña event, ocean water from off the coast of South America to the central tropical Pacific cools to below average temperatures. This cooling occurs because of stronger than normal easterly trade winds, which churns cooler, deeper sea water up to the ocean's surface. Sea temperatures can warm above average in the far western Pacific when this happens.